When you say Gollum's secret passage do you mean the tunnels through which Gandalf and the dwarves and then Bilbo ran through when escaping the goblins? If so then they probably wouldn't have been able to go through there because of the goblins, and also wouldn't that have taken them in the wrong direction?
And Tolkien did actually think ahead as he wrote. In HoME you can see the plot outlines that he wrote and he had forseen at least the basic plot of Lothlorien before they even entered Moria.
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